Data storage devices such as disk drives, tape drives, and solid state drives typically employ some form of error correction code (ECC) capable of correcting errors when reading the recorded data from the storage medium, thereby compensating for signal noise that is inherent with every recording/reproduction channel. During a write operation, redundancy symbols are generated and appended to user data which are then processed during a corresponding read operation in order to detect and correct errors in the user data. The number of errors that can be corrected increases as the number of redundancy symbols increases, but increasing the redundancy symbols also decreases the capacity of the storage medium. For example, with a Reed Solomon code employing N redundancy symbols, up to N symbols in error may be detected in a codeword, and up to N/2 symbols may be corrected. When the location of the erroneous symbols within a codeword are known (referred to as erasures), a Reed Solomon code is capable if correcting up to N symbols in error. That is, erasures assist the decoding of a Reed Solomon codeword as long as the number of erasures does not exceed the number of redundancy symbols in the codeword. If the number of erasures exceeds the correction power of the ECC, the codeword will fail to decode. This is true for essentially every ECC system, including an iterative ECC system such as a low density parity check (LDPC) code.
FIG. 1A shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Data tracks are defined relative to the servo tracks at the same or different radial density, wherein each data track comprises a plurality of data sectors. Each data sector may store the symbols of a single codeword, or in other embodiments, each data sector may store symbols from multiple codewords (i.e., interleaved codewords). FIG. 1B shows a prior art die format for a solid state drive, wherein each die may store multiple pages and each page may store multiple blocks each corresponding to a data sector of a disk drive.